Power inverters convert a DC power to an AC power. For example, some power inverters are configured to convert the DC power to an AC power suitable for supplying energy to an AC grid and, in some cases, an AC load that may or may not be coupled to the AC grid. One particular application for such power inverters is the conversion of DC power generated by an alternative energy source, such as photovoltaic cells (“PV cells” or “solar cells”), fuel cells, DC wind turbine, DC water turbine, and other DC power sources, to a single-phase AC power for delivery to the AC grid at the grid frequency. The amount of power that can be delivered by certain alternative energy sources, such as PV cells, may vary in magnitude over time due to temporal variations in operating conditions. For example, the output of a typical PV cell will vary as a function of variations in sunlight intensity, angle of incidence of sunlight, ambient temperature and other factors.
Such power inverters typically include a DC-DC converter to step up the voltage from a relatively low DC voltage (e.g., 30 V) to a power bus voltage of the inverter (e.g., 400 V). Depending on the particular implementation, the converter may be designed with any one of a number of different topologies. Common topologies include, for example, an isolated boost converter design, a “flyback” converter design, and a “series-LLC” converter design, each of which have distinct operational characteristics and/or components. In particular, the isolated boost converter is a “hard-switched” converter (i.e., the switches and diodes simultaneously experience a high current and high voltage stress during a switching transition) and often includes an active clamp circuit across a switch bridge to absorb mismatched current and limit the voltage. The flyback design is a generally simple converter topology having relatively simple control and gate drive requirements but has significant switch stresses and is an inherently hard-switched design. The series-LLC converter has an inherently soft-switched design and low voltage stresses and typically does not require an active clamp circuit. Unlike isolated boost converters, which are typically current-fed, series-LLC converters are fundamentally voltage-fed; because the input voltage generated by the PV cells may vary significantly over time, series-LLC converters must account for such variation. Accordingly, voltage-fed topologies tend to “step down” the voltage prior to boosting voltage (e.g., via a turns ratio of the transformer), which can result in inefficiency for the converter.
In a typical photovoltaic power system, an inverter may be associated with one or more solar cell panels. For example, some systems include strings of solar cell panels that deliver a relatively high, combined voltage (e.g., nominal 450 V) to a single, large inverter. Alternatively, in other systems such as a distributed photovoltaic power system, an inverter may be associated with each solar cell panel. In such systems, the solar cell panels are typically small, relatively low voltage (e.g., 25 V). The inverter may be placed in close proximity to the associated solar cell panel to increase the conversion efficiency of the overall system.